Compliant glass seal developmentat PNNL

12th SECA Workshop, Pittsburgh, PA, July 26-28, 2011

Y-S Matt Chou, E. Thomsen, J-P Choi, W. Voldrich,

and J. W. Stevenson

Introduction and objectives

Experimental: materials and test fixture

Q1: chemical compatibility study with YSZ coating

Q2: electrical stability under 0.8V loading

Q3: volatility evaluation in dual environment

Q4: validation in stack test fixture

This work is funded by US DOE SECA Core Technology Program

Compliant versus refractory sealing glass

2

Compliant sealing glass

0.00E+00

2.00E-03

4.00E-03

6.00E-03

8.00E-03

1.00E-02

1.20E-02

0 200 400 600 800 1000

stra

intemperature (oC)

YSO1 glass

Refractory sealing glass

Advantage: Disadvantage:

Low stress or relaxation, healing? metal-stable, narrow T window,

Wetting, volatile, reactive/corrosive?

Data provided by ORNL

850h aged

s = E a T

Crack healing

Commonly observed in glass at elevated temperatures.

3 mechanisms proposed: diffusion-driven thermal healing, adhesion from intermolecular forces, and chemical reaction at crack-tip

3

SCN-1 indented @ 2 kg Fired to 700oC held for 6min

Objectives

To conduct a comprehensive study of a commercial

compliant sealing glass in terms of thermal, chemical,

electrical, physical, and mechanical stability in SOFC

environments.

To apply compliant glass in stack test fixture for

validation.

4

Experimental: sample preparation

5

1. Thickness of spacer rings ~220 mm

2. SCN-1 glass mixed with ESL450 binder to form paste

AISI441

PEN

YSZ spacer ringsglass

Machined groove design

Spacer ring design

6

Experimental: stability and high-temp leak test

6

PEN cell or bilayer (1.4”f)

Al2O3 pipe

porous Al2O3 support

Inconel600 load block

(2”x2”) with incoming and

outgoing tubing

SCN-1 orAg/mica hybrid seal

with known leak rate

or plain Ag foils

Load (~12 psi)

He/H2 inHe/H2 out or to

pressure sensor

Compliant glass

Corrugated Ni-mesh

to mimic some

contact load

Al-SS441 (2”x2”x0.06”

with a central hole)

ZrO2 or mica ring

spacer

Microstructure of YSZ and Al2O3 coated SS441

7

Fe

Cr

Al

Zr

Q1: chemical compatibility with YSZ and Al2O3coated SS441 in dual environment at 750oC

8

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0 200 400 600 800 1,000

Leak

Ra

te (s

ccm

/cm

)

Time (Hours) @ 750oC

Test #35 Zirconia Coated 441 with machined groove SCN-1 sealed at 800°C/2hr leak rate

@ 750°C in 5% H2 perimeter sealed with Ag

0.2psi

0.5psi

1psi

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0 2 4 6 8 10 12

Leak

Ra

te (s

ccm

/cm

)

# deep thermal cycle

Test #35 Zirconia Coated 441 with machined groove SCN-1 sealed at 800°C/2hr leak rate

@ 750°C in 5% H2 perimeter sealed with Ag

0.2psi

0.5psi

1psi

Leak test during isothermal ageing Leak test during thermal cycling

SCN-1 glass showed good thermal and thermal cycle stability at 700, 750,and 800oC

Post-mortem analysis: glass/YSZ electrolyte interface near air side

9

Zr Si

KNa Ba

Post-mortem analysis: glass/YSZ coating interface near air side

10

SS441

Fe

Cr

Zr

Si

K

Na

Minimal crystallization during pure thermal cycling

11

800oC 20 cycles 750oC 20 cycles

Ceramic bi-layer

Ceramic bi-layer

Aluminized-441Aluminized-441

More crystallization during isothermal ageing

12

800oC/1000h plus 10 cycles 750oC/1000h plus 10 cycles

Ceramic bi-layer

Ceramic bi-layer

More crystallization during isothermal ageing

13

700oC/1000h plus 10 cycles: Ba-silicate and K-Al silicate

oxide mole%

Al2O3 1.84

BaO 3.57

CaO 3.96

Fe2O3 0.09

K2O 7.07

MgO 1.03

Na2O 7.83

TiO2 0.45

ZnO 0.01

ZrO2 0.01

Li2O 0.05

B2O3 0.03

SiO2 74.06

Alu

min

ized-4

41

Cera

mic

bi-la

yer

14

Q2: Electrical stability test

14

VV

VV

resistor

SS441

SS441

Inconel

Al2

O3

5%H2 in

+

+

-

-

sense

sense

out

out

+

+

-

-

sense

sense

out

Hybrid mica

SCN glassPower supply

And sensor

load

5%H2 out

15

Chemical compositions

15

SCN-1 glass Refractory glass

YSO77

oxide mole%

Y2O3 6.00

BaO 6.00

SrO 42.50

B2O3 10.00

SiO2 35.50

oxide mole%

Al2O3 1.84

BaO 3.57

CaO 3.96

Fe2O3 0.09

K2O 7.07

MgO 1.03

Na2O 7.83

TiO2 0.45

ZnO 0.01

ZrO2 0.01

Li2O 0.05

B2O3 0.03

SiO2 74.06

Alkali ions (Na+ and K+) are major charge carriers for conductivity in silicate glasses

Two concerns: liquid glassy phases, and alkali effect

16

Comparison of crystallized glass (YSO75) and less crystallized glass (G18) with as-received crofer

16

YSO75 @0.7V & 20%H2O, 2.7%H2/Ar (t<432hrs), 80%H2 (t>432hrs)

power outage @ ~700 h

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

0 200 400 600 800 1000 1200

hrs at elevated temperatures

oh

m-m

G18 @830oC

YSO75 @ 850oC20%H2O, 2.7%H2/Ar

20%H2O, pure H2

G18 (less crystallized compared to refractory glass) showed rapid decrease

In resistivity, suggesting dissolution/diffusion of metal ions from crofer22APU

17

Electrical stability with YSZ coated Aluminized SS441

17

Tested with flowing 5%H2 (~3%H2O) and a DC load of 0.8V across glass

Stable apparent resistivity (calculated by ohm x area/thickness neglecting coating

contribution) indicates coatings remain intact (compatible) with SCN-1 glass.

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

0 200 400 600 800 1000

resi

stiv

ity,

oh

m-m

hours

stability of YSZ coated aluminized SS441

800C

750C

700C

Microstructure of 800oC/1000h electrically tested sample with YSZ coating

18

YS

Z-4

41

YS

Z-4

41

glass

Chemical compatible with YSZ coating

19

No reaction and segregation of alkalis at interfaces under DC loading

SiZr

K

NaAl

electrical stability with plain SS441

20

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

0 100 200 300 400 500

Ap

pa

ren

t re

sist

ivit

y, o

hm

-m

hrs

SCN glass with as-received SS441 @0.8V 800C

750C

700C

Presence of Fe in SCN-1 glass

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spot # O Na Mg Al Si K Ca Ti Cr Mn Fe Ba

1 67.09 26.46 0.35 4.90 1.20

2 65.41 13.52 0.84 19.43 0.51 0.29

3 66.89 0.63 0.25 1.55 27.67 1.92 0.14 0.14 0.53 0.29

4 64.84 2.24 0.61 1.54 24.90 3.32 0.86 0.70 1.00

5 65.83 0.95 0.31 1.28 28.26 2.04 0.37 0.10 0.38 0.48

6 64.44 2.43 0.69 1.25 24.84 3.17 1.23 0.15 0.50 1.31

12

34 (Fe=0.7%)

5

6 (Fe=0.5%)

Fe at%=0.06% SCN-1

22

Electrical stability with Aluminized SS441

22

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

0 100 200 300 400 500

resi

stiv

ity,

oh

m-m

hrs

SCN with aluminized SS441 @0.8V

800C

750C

700C

Tested with flowing 5%H2 (~3%H2O) and a DC load of 0.8V across glass

Stable apparent resistivity indicates coatings remain intact (compatible) with SCN-

1 glass.

Q3: volatility issue

23

Weight loss vs. time data provided by ORNL of SCN-1 pellets at 800oC in

stagnant air or flowing steam

Linear volatility rate (R) was averaged for 5000h (steam) or 10000h (air)

Test cond.

800oC

Avg. volatility

(g/mm2-hr)

Total wt. loss (%)

in 40,000h

YSZ-air 6.33x10-9 1.7

YSZ-steam 9.90x10-10 0.3

Al2O3-air 8.04x10-9 2.1

Al2O3-steam 1.08x10-9 0.3

Total weight loss %

= (R*L*T*40,000*100)/(r*L*T*W)

Q4: evaluate glass in stack test fixtures-thermal cycle stability

Objectives: thermal cycle stability, containment/spreading issue, and volatile species interaction in base-line stability for 1000h

Reasonable consistent OCV at 800oC (1.040 V) theoretical 1.048V over 10 deep thermal cycles (~50oC to 800oC in 3h, 800oC/3h, 18h to ~50oC).

24

0.7

0.8

0.9

1.0

1.1

0 50 100 150 200 250

OC

V, V

hrs

225H2, 225N2, 800 air, 31oC H2O

Q4: evaluate glass in stack test fixtures-containment/spreading issue

25

WF

PENFuel side

air side

glassFuel side Air side

glass

glass

Summary and conclusion

Compliant SCN-1 glass was evaluated comprehensively in thermal

cycles stability, thermal stability, and chemical compatibility with

candidate SOFC materials.

The glass demonstrated very good thermal cycle and thermal stability

with constant leak rate.

Interfacial characterization showed minimum reaction at YSZ

electrolyte and YSZ coating interfaces.

Electrical stability test under DC load indicated insulating nature of the

glass and no segregation of alkalis. Crystallization was similar as

1000h stability test without DC loading.

Volatility estimation indicated minimal glass loss in 40,000h operation.

Preliminary evaluation in stack test fixture showed good thermal cycle

stability without containment/spreading issue.

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Future work

Optimize YSZ coating in terms of thickness with collaboration with modeling to stress minimization.

Evaluate stability (1000h) in stack test fixtures at various temperatures with emphasis on interaction of volatile species with active SOFC components.

Modify processing and sealing conditions to eliminate large pore formation.

Collaborate with modeling to predict microstructure evolution effect on compliance, thermal and physical properties.

Continue evaluating the spreading/containment issue, and predict the life-time flow under differential pressure.

Design new window frame for robust sealing.

27